1. Field of the Invention
This invention relates generally to the field of semiconductor device manufacturing and, more particularly, to a method and apparatus for scheduling metrology based on a jeopardy count.
2. Description of the Related Art
To fabricate a semiconductor device, a wafer is typically processed through numerous processing tools in a predetermined sequence. The processing tools may include photolithography steppers, etch tools, deposition tools, polishing tools, rapid thermal anneal tools, ion implantation tools, and the like. Each processing tool modifies the wafer according to a particular operating recipe. For example, a photolithography stepper may be used to form a patterned layer of photoresist above the wafer. Features in the patterned layer of photoresist correspond to a plurality of features, e.g. gate electrode structures, which will ultimately be formed above the surface of the wafer. The tool sequence, as well as the recipes used by the tools, must be carefully controlled so that the features formed on the wafer meet appropriate design and performance criteria. Thus, advanced process control (APC) systems are often used to coordinate operation of the processing tools.
Processed wafers are provided to metrology tools at various stages of the processing. The metrology tools perform measurements on the processed wafers and the measured parameters may be used to determine to assess the condition of the processing tools, among other things. If the measurement parameters are within predetermined tolerances, then it is generally assumed that the tools used to process of the wafer are functioning in a desired and/or expected manner. For example, a wafer that has been processed in a deposition tool to form a layer on the wafer (or above another layer or other features formed on the wafer) may be provided to an ellipsometer to determine a thickness of the layer. If the ellipsometer determines that the thickness of the layer formed by the deposition tool is within a tolerance of a desired and/or expected thickness, the ellipsometer is generally assumed to be functioning in a desired and/or expected manner. If the measurement parameters are outside the predetermined tolerances, then the tools may not be functioning in the desired and/or expected manner.
Conventional semiconductor processing systems often include many more processing tools than metrology tools. Consequently, only a portion of the processed wafers may be provided to a metrology tool and the processed wafers that are provided to a metrology tool may be forced to wait in a queue, such as a first-in-first-out (FIFO) queue, before being provided to the metrology tool. The discrepancy between the (relatively fast) rate at which wafers may be processed in processing tools and the (relatively slow) rate at which wafers may be provided to metrology tools for analysis may result in wafers being processed in processing tools that may not be functioning in a desired and/or expected manner. For example, a processing tool may begin to operate in an undesirable manner while wafers processed by the processing tool are waiting in a metrology queue. Thus, the undesirable operation of the processing tool may not be detected by metrology until the wafers in the queue can be provided to the metrology tool, resulting in wafers being processed in an undesirable manner.
Conventional advanced process control systems typically maintain a jeopardy count, at least in part to limit the number of wafers that may be processed by processing tools operating in an undesirable and/or expected manner. The jeopardy count indicates how many wafers have been processed by a tool (or set or tools) since the last metrology was performed on a wafer processed by the tool. For example, a processing tool that has processed 10 wafers since the last wafer processed by the processing tool was provided to a metrology tool would have a jeopardy count of 10. The jeopardy counts may be compared to a critical jeopardy count and processing by the processing tool may be halted if the jeopardy count of the processing tool surpasses the critical jeopardy count. For example, if the critical jeopardy count of a processing tool is 10, then processing by the processing tool may be halted when the processing tool has processed more than 10 wafers since the last metrology check. Jeopardy exceptions such as these may reduce the efficiency of the semiconductor processing system.
The present invention is directed to addressing the effects of one or more of the problems set forth above.